Would be really cool if all customizations would really matter and do something. Like in addition for supressor supressing with downside of lowering damage outcome, and optics making you see far with cost of peripherial vision (when looking down the sights), other things would have their bonuses and maluses. Stocks could change the accuracy but make weapon little slower to move around; barrels could do something with recoil, damage and accuracy... Again, would be really interesting if weapons would have kind of balance\weight factor, depending on weight and positions(where possible, as for rail systems) of different attachments affecting recoil and weapon's movement...
Cause there is already a lot of games with people running around with overpimped guns... But again, just a suggestion\my_view_on_that, i dont yet quite understand what are you aiming for in yor game.